This invention relates to an elastic surface wave device using an X-cut lithium tantalate (LiTaO.sub.3) substrate.
As a substrate of an elastic surface wave device one may make use of a Y-cut Z-propagating LiNbO.sub.3 substrate, a 130.degree. rotated Y-cut LiNbO.sub.3 substrate, a piezoelectric ceramic, a Y-cut Z-progagation LiTaO.sub.3 substrate, an ST-cut quartz device, etc. based on the IRE (Institute of Radio engineering) standard.
The above-mentioned known piezoelectric devices have the following drawbacks. The Y-cut Z-propagating LiNbO.sub.3 device has a sufficiently great piezoelectric coupling coefficient and relatively small dielectric constant and it finds general application. However, the temperature drift of the surface wave velocity has a value of about 80 PPM/C.degree. and the temperature characteristic is greatly lowered. An interdigital electrode on the reception side receives a bulk wave and when such a LiNbO.sub.3 device is used, for example, as a filter, a high bulk spurious response occurs at the upper area of its passband.
Although the 131.degree. rotated Y-cut LiNbO.sub.3 device has a better coupling coefficient, dielectric constant and bulk spurious characteristic, the temperature coefficient still has a high value of about 80 PPM/C.degree..
A piezoelectric ceramics device has been developed to improve the temperature characteristic. However, it has a high dielectric constant and a high variation in surface wave velocity. The Y-cut Z-propagation LiTaO.sub.3 device has a high temperature coefficient.
The ST-cut quartz device has a better temperature coefficient, but a lower coupling coefficient.
As will be understood from the above, the abovementioned known piezoelectric bodies have various merits and demerits. An elastic surface wave device having sufficiently desirable overall characteristics has not been offered to date.